Thank you very much Finally some1 have given me a direct answer..
If I upgraded the CPU to Q9400 along with the vga, how much would that effect the performance with C2Q games?? I already played GTA IV but had to use very low resolution and settings, Haven't played Crysis yet cause I just didn't waste such a great game with low resolution and setting..
You might want to read this THG article on building a balanced PC with a lot of charts showing the impact of upgrading either or both CPU and Graphics card. Since you mentioned the GTA, I linked to the page showing that test:
Note that the two closest CPUs to the ones you are considering are the E6300 and Q9550 so you will have to interpolate a little, but you can see the substantail gains from increasing the CPU. Also the 5770 is not in the sample - it should fall about midway between the 4850 and 4890 - very similar to the GTX-260.
I think you should consider getting a good video card now - even if you don't upgrade the CPU - because you can simply move it to your new system - and maybe later pair it with another card for even faster perfomance.
However if you want a stopgap card now, Fry's has a great sale on a BFG GTS 250 1GB graphics card for only $99.
Thank you guys very much for your detailed suggestions and inputs, I will definitely keep in mind all what you said when I upgrade in the next few days.
I guess I shouldn't spend much money right now upgrading this old rig other than the GPU since I will only use it for just 1 more year then will get a new i7 system with 5970 or whatever is better 1 year from now.